Method of surface finishing of metal moldings

ABSTRACT

A method for producing a vehicle wheel having a polished design surface coated with a transparent coating, comprising the following steps of: producing a vehicle wheel having a design surface thereon; and coating said design surface with transparent composite material which is mainly composed of inorganic material.

This is a divisional of application Ser. No. 08/435,212 filed May 5, 1995, which is a divisional of application Ser. No. 08/082,295 filed Jun. 28, 1993 now U.S. Pat. No. 5,491,030.

BACKGROUND OF THE INVENTION

The present invention relates to a method of finishing surfaces of metal molded products. The present invention also relates to a method of producing a vehicle wheel.

If metal products are made to have well finished surfaces in a bright state, the value added of the metal products becomes high, and the surface finishing has been performed manually by hands from of old.

The today's industrial mass production technique is however premised on use of various working machines, and when a decision is made as to a manner how to finish the surfaces of metal products, it is an important factor whether the finishing method has been established as a mass-production technique using a working machine or not.

In such a background, with respect to surface finishing of metal products, a mechanical mass-production technique for making plate materials or the like having flat surfaces have extremely well finished surfaces has been established and carried out widely.

With respect to the surfaces of molded products such as cast products, forged products, or the like, however, uneven portions exist on the surfaces so that it is difficult to mechanically well finish the surfaces, particularly the concave portions portions thereof, and it is the existing circumstances that the finishing is performed not mechanically but mainly manually by hands.

On the other hand, since vehicle wheels constitute the appearance of a vehicle, the appearance of design surfaces of the wheels is regarded to be extremely important from the point of view of a commodity value.

There is therefore a conventional method in which a design surface of a vehicle wheel formed through molding such as casting or forging is finished bright in order to improve the appearance thereof.

Since such a vehicle wheel has a fear to be damaged by flying stones or the like during running of the vehicle, a transparent coating is formed on the brightly finished design surface so that the design surface can be protected from being injured.

However, since such a transparent coating cannot be perfectly transparent, the brightness of the design surface cannot appear as it is, that is, this transparent coating sacrifices the brightness of the design surface at a some extent.

In such surroundings, conventionally, the finishing of such a design surface is made so that the finished design surface has a bright state to an extent that the bright state can be recognized by human eyes through a transparent coating formed on the surface.

Recently the transparency of transparent coatings has been improved with the advance in technique with respect to the materials etc. for such transparent coatings. Adversely, however, there occurs a contradictory situation in which minute injuries on a design surface per se caused by finishing, or unevenness of the finishing can be recognized through eyes as a result of the improvement in transparency of the transparent coatings, so that the improvement of the transparency of transparent coatings may lower the appearance of such vehicle wheels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a step explanation diagram of a whole process of finishing surface.

FIG. 2 is an explanation diagram of a polishing step in the steps of preliminary and mirror finishing.

FIG. 3 is an explanation top view of a step of longitudinal-axis wet buffing.

FIG. 4 is a cross-sectional view taken on line 4--4 of FIG. 3.

FIG. 5 is an explanation top view of a step of transversal-axis wet buffing.

FIG. 6 is a cross-sectional view taken on line 6--6 of FIG. 5.

FIG. 7 is an explanation diagram of the contact state of a buff to a spoke portion in a step of transversal-axis wet buffing.

FIG. 8 is an explanation diagram of a step of electrolysis composite-type longitudinal-axis wet buffing, which is a cross-section taken on line 8--8 of FIG. 3.

FIG. 9 is a cross-sectional view of a design surface on which a transparent coating is formed.

FIG. 10 is a timing chart showing the operation to execute a wet buffing method according to the present invention.

FIG. 11 is a timing chart showing a modification of that of FIG. 10.

FIG. 12 is a cross sectional view showing a buff.

FIGS. 13A and 13B are explanatory diagrams the buffing operations by the buff of FIG. 12.

FIG. 14 is a cross sectional view showing a modification of the buff of FIG. 12.

FIG. 15 is a cross sectional view showing another embodiment of the invention.

FIG. 16 is a diagram showing the metal structure of the surface of an aluminum alloy blank.

FIG. 17 is a cross sectional view showing a model of a part of the cross section of the metal structure shown in FIG. 16.

FIG. 18 is an explanatory diagram for explaining the relationship between DAS2 of the aluminum alloy blank after mirror finished and surface roughness.

FIG. 19 is a side view, partly in cross section, showing how aluminum alloy blank as a test piece is barrel worked.

FIG. 20 is an explanatory diagram useful in explaining the method of measuring DAS2 measuring method based on the secondary technique.

FIG. 21 is a front view showing a vehicle wheel.

FIG. 22 is a cross sectional view taken on line 22--22 in FIG. 21.

FIG. 23 is a side elevational view showing a vehicle.

SUMMARY OF THE INVENTION

In order to attain the foregoing object, the invention provides a method of surface finishing of metal molded products, characterized in that a surface finishing step finally performed on a metal molded product includes a first polishing step in which a buffing device having a transversal rotation axis and a soft buff is used so that the circumferential surface of the buff is made to come into contact with the surface of the metal molded product while supplying an abrasive liquid containing abrasive added thereto.

The present invention has been also attained based on the foregoing circumstances, and an object thereof is to provide a surface finishing method for mechanically finishing the surfaces of metal molded products which have been finished mainly manually by hands so that the molded products have extremely well finished surfaces.

According to a primary aspect of the invention, there is provided a method for producing a vehicle wheel having a polished design surface coated with a transparent coating, comprising the following steps of:

producing a vehicle wheel having a design surface thereon; and

coating said design surface with transparent composite material which is mainly composed of inorganic material.

A surface roughness on said design surface of said vehicle wheel is preferably not greater than 3pm in terms of centerline average roughness.

The vehicle wheel comprises aluminum alloy containing silicon.

According to the present invention, there is also provided a vehicle wheel having a polished design surface coated with a transparent coating, wherein the vehicle wheel is coated with transparent composite material which is mainly composed of inorganic material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereunder, the present invention will be described with reference to an embodiment illustrated in the drawings. In this embodiment, the technique of the present invention is applied to light-alloy articles such as vehicle wheels which are cast products of an aluminum alloy. In this specification, the design surface means a surface which faces outward from a vehicle body and forms a part of appearance of a vehicle, when the wheel is mounted on the vehicle.

First, a vehicle wheel blank 1 as a molded product is a cast product obtained by pouring an molten aluminum alloy (for example, AC4CH) alloy (containing silicon) into a mold having a predetermined shape.

The vehicle wheel blank 1 obtained thus is taken out of the mold and subjected to a process of shot peening after removal of gate portions.

The working conditions in this shot peening is established, for example, as shown in Table. 1.

                  TABLE 1                                                          ______________________________________                                         Kind of Shot     steel balls or steel wire chips                               Size of Shot     equivalent to 0.8 to 1.2 mmφ                              Spraying         air spraying                                                  Air Pressure     5.2 to 6.0 kg/cm.sup.2                                        Speed of Spraying                                                                               not less than 80 m/sec                                        Quantity of Projection                                                                          400 to 600 kg/min                                             ______________________________________                                    

Although shot peening is performed all over the vehicle wheel blank 1 for 20 minutes under such conditions, shot peening in the portion of a design surface 2 of the vehicle wheel blank 1 is performed specially carefully since a transparent coating 31 is to be formed on the design surface in this embodiment as will be described later.

Shot peening is a process having a comparatively high degree of freedom, so that it is possible to spray shots almost uniformly all over the curved design surface 2 to thereby form a plastic deformation layer uniformly.

The design surface 2 of the vehicle wheel blank 1 subjected to such shot peening becomes a minute matte-finished rough surface, and a plastic deformation layer is formed in its superficial portion.

Since this plastic deformation layer is formed by the impact of shots, the plastic deformation layer opens to the design surface 2 of the vehicle wheel blank 1, so that holes or cracks such as bubbles containing air or moisture therein have disappeared by being compressed in the plastic deformation layer, and air contained in such holes, water used in the working of casting the vehicle wheel blank 1, and so on, had been exhausted outside.

Particularly shot peening in this embodiment has a high speed of spraying a shot, that is, high kinetic energy of a shot as mentioned above, so that the thickness of the plastic deformation layer is large about 1.5-P to 3.0-P, so that the plastic deformation layer formed in the superficial portion of this design surface 2 is substantially free of air or moisture.

Therefore, even if preliminary finishing or mirror finishing, which will be described later, is performed by wet buffing, there is no fear that moisture resides in the superficial portion of the design surface 2 so as to become a hindrance to the formation of the transparent coating 31.

Needless to say, such shot peening can be performed under any conditions other than those mentioned above.

Further, since such shot peening is to spray shots to impact the surface of the vehicle wheel blank 1 to be worked, it is possible to remove and eliminate mill scale, adhering foreign matters, dirt or the like, from the surface of the vehicle wheel blank 1 to be worked to thereby make the surface clean.

Then, the vehicle wheel blank 1 is machined in its predetermined portion in a step of machining so as to be formed into a predetermined accurate shape as the vehicle wheel blank 1.

The design surface 2 of the vehicle wheel blank 1 is then subjected to various steps shown in FIG. 1 so that a vehicle wheel having the design surface 2 finished into a bright mirror-finished surface can be obtained as a product at last.

In the vehicle wheel blank 1 brought into the above-mentioned state, a number of uneven portions are formed by spoke portions 3, through holes 4, hub portions 5, rim portions 6 and so on from the request of design (with reference to FIGS. 3 and 4). Mill scale is left to a some extent on this design surface 2.

First, a step of removing mill scale is performed upon the design surface 2 having such residual mill scale.

The step of removing mill scale in this embodiment is performed gradually with a grinding machine by use of abrasive paper mainly having abrasive #100 to #320. In accordance with the size and shape of the roughness of the design surface 2, a rotor having the same size may be used as auxiliary, or hand polishing may be carried out in addition thereto.

The method of removing mill scale is not limited to that using such abrasive paper, but any other method may be used if it is possible to obtain a finished surface to the same extent.

A step of rough finishing as follows is performed upon the design surface 2 of the vehicle wheel blank 1 from which mill scale has been removed by such a step of removing mill scale.

This step of rough finishing is performed by so-called dry buffing without using any abrasive liquid through divisional stages, for example, two stages.

That is, the first stage of this rough finishing step is performed at a high rotation speed, for example, of about 2000 rpm by use of a sisal buff and a half finishing compound.

The second stage of the rough finishing step is performed at the same rotation speed as mentioned above by use of a cotton buff having a smaller rigidity than the above-mentioned sisal buff, and an extremely superior finishing compound.

Upon completion of such a step of rough finishing, the design surface 2 has the finished surface equivalent to those conventionally attained on molded products in mass only by using a machine.

This embodiment is directed to perform a step of preliminary finishing and a step of mirror finishing, as follow, upon the design surface 2 of the vehicle wheel blank 1 subjected to such a step of rough finishing, so that it is possible to mechanically attain mirror finishing which has been realized on a molded product only by hand working by a skilled worker.

These steps of preliminary finishing and mirror finishing are carried out by so-called wet buffing with an abrasive liquid through 5 divisional stages in this embodiment as shown in FIG. 2.

First, as the first stage of the step of preliminary finishing, a step of electrolysis composite-type longitudinal-axis wet buffing is performed upon the design surface 2 of the vehicle wheel blank 1 which has been finished thus by the above-mentioned rough finishing consisting of dry buffing.

The contents of the step of electrolysis composite-type longitudinal-axis wet buffing are as follows (with reference to FIGS. 8, 3 and 4).

That is, in this step of electrolysis composite-type longitudinal-axis wet buffing, the vehicle wheel blank 1 is fixed on a rotary table 11 so that its center is made to coincide with the rotation center O of the rotary table 11 and the design surface 2 is fixed to its upward and horizontal attitude. Then, the design surface 2 is polished by the lower end surface of a buff 13 attached to the lower end portion of a rotation shaft 12 hung downwards.

The rotary table 11 is driven to rotate at a low speed at the time of working and the portion to be worked by the buff 13 is displaced circumferentially sequentially. In this embodiment, the rotating direction of the rotary table 11 is reversed at desired time intervals (for example, 30 seconds).

The buff 13 is formed of an extremely soft material in comparison with a conventional buff, for example in this embodiment, a continuous bubble sponge material having a large expansion ratio (for example, 3 to 5). The buff 13 is not limited to a sponge material, but any other material may be used as the buff 13 as long as it is as soft as the sponge material.

Therefore, the lower end surface of the buff 13 can keep contacting with the design surface 2 surely in spite of the presence of the uneven portions of the design surface 2 as shown in FIG. 4.

The urging force of the buff 13 against the design surface 2 is small, for example, not larger than 1 kgf/cm².

An electrode 14 having a shape corresponding to the shape in plan of the buff 13 is provided in the upper portion of the buff 13 (with reference to FIG. 8), so that this electrode 14 is used for performing an electrolytic working function which will be described later.

The rotation shaft 12 supporting the buff 13 in its lower end portion is held by an elevator 15 constituted by an oil pressure cylinder and so on. The rotation shaft 12 is driven, for example, at a rotation speed of 300 rpm so that the rotation speed of the buff 13 is made to be extremely low not higher than about 10 m/sec, for example, 4 m/sec in this embodiment.

This is because the above-mentioned soft material constituting the buff can be deformed surely following the uneven portions of the design surface 2, while keeping its contact state with the uneven portions.

This rotation shaft 12 is driven to rotate in the direction which is coincident with the rotating direction of the rotary table 11, and if the rotating direction of the rotary table 11 is reversed to the opposite direction, the rotation shaft 12 is pulled up by the elevator 15 so that the buff 13 moves upward away from the design surface 2, and the rotation shaft 12 is driven to rotate in the opposite direction at the same time.

Therefore, even if the rotating direction of the rotary table 11 is reversed, the respective rotating directions of the rotary table 11 and the buff 13 are always kept to be coincident with each other.

A shaft hole 12a is formed in the rotation shaft 12 (FIG. 8), so that an abrasive liquid containing abrasive added thereto is supplied to the center portion of the buff 13 through this shaft hole 12a. This abrasive liquid has a function as electrolyte in electrolytic working which will be described later, and the abrasive liquid is an aqueous solution containing alumina of 10 μm in particle diameter as abrasive in this embodiment, and containing also a surface active agent added thereto.

Further, an abrasive liquid nozzle 16 for supplying the same abrasive liquid as mentioned above to a contact portion between the buff 13 and the design surface 2 is provided to supply fresh abrasive to the buff 13 surely as shown by a phantom line in FIG. 3.

Since supply of the abrasive liquid from this abrasive liquid nozzle 16 is continued even when the rotating direction of the rotary table 11 is reversed, the abrasive liquid is sprayed to a portion which has been directly under the buff 13, when the buff is rising up in accordance with the reversal of the rotating direction of the rotary table 11, so that it is advantageously possible to effectively eliminate buffing dusts and other foreign matters which have been at a position between the buff 13 and the design surface 2.

Then electrolysis working in this step of electrolysis composite-type longitudinal-axis wet buffing is performed in combination with the above-mentioned longitudinal-axis wet buffing as shown in FIG. 8.

That is, electrolysis working in this embodiment is performed with a DC power supply 17 interposed between the electrode 14 and the vehicle wheel blank 1 and the above-mentioned abrasive liquid, and the electrode 14 and the design surface 2 of the vehicle wheel blank 1 are connected. to the positive and negative sides respectively.

The abrasive liquid supplied from the shaft hole 12a of the rotation shaft 12 and the abrasive liquid supplied from the abrasive liquid nozzle 16 are present between the electrode 14 and the design surface 2 penetrating the buff 13 formed of a continuous bubble sponge material, so that an electrolytic circuit from the DC power supply 17 is formed therebetween by such an abrasive liquid, so that electrolysis working is performed in the surface of the design surface 2 at the same time as the above-mentioned buffing with abrasive,so that electrolysis composite-type buffing is performed.

In this step of electrolysis composite-type longitudinal-axis wet buffing, therefore, the working efficiency is superior to that in ordinary buffing.

After completion of this step of electrolysis composite-type longitudinal-axis wet buffing, a step of transversal-axis wet buffing is performed on the design surface 2. In order to avoid confusion in the following description, this step of transversal-axis wet buffing will be referred to as a step of preliminary finishing transversal-axis wet buffing hereinafter.

The contents of this step of preliminary finishing transversal-axis wet buffing are as follows (with reference to FIGS. 5 to 7).

That is, also in this step of preliminary finishing transversal-axis wet buffing, the vehicle wheel blank 1 is fixed on a rotary table 21 so that its center is made coincident with the rotation center O of the rotary table 21. and the design surface 2 is fixed in its upward and horizontal attitude, and the design surface 2 is polished by the circumferential surfaces of buffs 23a and 23b attached to rotation shafts 22a and 22b elongated to the upper portion of the design surface 2 from both the right and left sides.

The rotary table 21 is driven to rotate only in a predetermined direction at a low speed at the time of working, while the portion worked by the buffs 23a and 23b is displaced sequentially in the the circumferential direction.

The buffs 23a and 23b are formed of an soft material, which is, in this embodiment, an independent bubble urethane foamed material having a large expansion ratio (for example, 3 to 5). Any other material may be used for these buffs 23a and 23b so long as it is as soft as the material mentioned above.

In this embodiment, the buffs 23a and 23b are made of an lamination of plurality (5 in the illustrated case) of sheets of buff material of the required thickness which are penetrated by the rotation shafts 22a and 22b, and the circumferential shape of each of the buffs 23a and 23b is made to approximate to the shape of the radial cross-section shape of the design surface 2 (with reference to FIG. 6).

Therefore, the buffs 23a and 23b can keep their circumferential surfaces having a linear contact with the design surface 2 in its radial direction surely in spite of the presence of uneven portions in the radial direction of the design surface 2.

As has been mentioned, since the buffs 23a and 23b have a linear contact with the design surface 2 in the step of preliminary finishing transversal-axis wet buffing, the polishing force by supplied abrasive is large as will be described later, so that it is possible to effectively delete electrolysis traces caused by the above-mentioned step of electrolysis composite-type longitudinal-axis wet buffing.

Further, these buffs 23a and 23b have transversal axes, and their contact portions with the design surface 2 are circular arcs, so that it is comparatively easy to intrude the buffs 23a and 23b deeply into concave portions formed on the design surface 2 to polish the inside of the concave portions.

Since these buffs 23a and 23b have a linear contact with the design surface 2, if the buffs 23a and 23b are set onto the design surface 2 to be given flexibility in advance, the circumferential surfaces of the buffs 23a and 23b can contact with the concave portions on the design surface 2 surely by releasing the flexibility as will be described later.

Also in this step of preliminary finishing transversal-axis wet buffing, the rotation speed of the rotation shafts 22a and 22b is, for example, 100 to 500 rpm, and a low speed of 0.8 to 10 m/sec so that the soft buffs 23a and 23b are intended to contact into the concave portions surely by releasing their flexibility in the same manner as in the step of electrolysis composite-type longitudinal-axis wet buffing.

In this embodiment, the rotation shaft 22a is driven to rotate in the direction coincident with the rotating direction of the rotary table 21, and opposite to the rotating direction of the other rotation shaft 22b.

Further, abrasive liquid nozzles 26a and 26b are provided respectively toward the portions in which the buffs 23a and 23b driven to rotate by the rotation shafts 22a and 22b starts contacting with the design surface 2.

The abrasive liquid sprayed and supplied from these abrasive liquid nozzles 26a and 26b is the same as that mentioned above, which is a solution including alumina of the particle diameter 10 μm as abrasive, and a surface active agent added thereto, to thereby supply fresh abrasive to the buffs 23a and 23b.

In such a step of transversal-axis wet buffing that the rotation shafts 22a and 22b are set to almost transversal attitude to perform buffing with the circumferential surfaces of the buffs 23a and 23b as in this step of preliminary finishing transversal-axis wet buffing, the contact areas between the buffs 23a and 23b and the design surface 2 are linear as has been described, so that it is not only possible to supply fresh abrasive into the contact areas surely by spraying from the abrasive liquid nozzles 26a and 26b, but it is also possible to easily wash away foreign matters such as buffing dusts by the sprayed abrasive liquid.

Consequently the design surface 2 can be made a superior finished surface corresponding to used abrasive, as follows, in spite of the unevenness thereof by this preliminary finishing transversal-axis wet buffing.

That is, if the buffs 23a and 23b are set to be given predetermined flexibility d as shown in FIG. 7, it is possible to make the lower ends of the circumferential surfaces of the respective buffs 23a and 23b reach to the lower portions correspondingly to the flexibility d in response to the release of the flexibility d.

This means that, if the depths of the concave portions are almost not more than the size d, the buffs 23a and 23b can contact with the inside of concave portions which exist in the circumferential direction of the design surface 2, so that wet buffing can be performed also in the concave portions by supplying an abrasive liquid.

Further, the reason why the buffs 23a and 23b having rotating directions opposite to each other are arranged so as to come into contact with the design surface 2 in this embodiment is that the contact of the buffs with the circumferential both-side portions of the spoke portion 3 elongated in the radial direction on the design surface 2 can be made uniform by driving the vehicle wheel blank 1 in one direction by means of the rotary table 21, so that buffing to the both sides of the spoke portion 3 can be made uniform.

Since working is performed while providing and releasing the flexibility d of these buffs 23a and 23b as has been described, the buffs 23a and 23b themselves generate heat, but advantageously the lives of the buffs 23a and 23b can be prolonged because an abrasive liquid is directly sprayed to the flexible portions of the buffs 23a and 23b as mentioned above.

Next a step of longitudinal-axis wet buffing is performed on the design surface 2 in which such a step of preliminary finishing transversal-axis wet buffing has been completed. In order to avoid confusion in the following description, this step of longitudinal-axis wet buffing will be referred as a step of preliminary finishing longitudinal-axis wet buffing.

This step of preliminary finishing longitudinal-axis wet buffing is performed by the same means (with reference to FIGS. 3 and 4) as that used in the above-mentioned step of electrolysis composite-type longitudinal-axis wet buffing, but it is only a different point that means concerning electrolysis working such as the electrode 14, the DC power supply 17, etc. (FIG. 8) are not provided.

The step of preliminary finishing longitudinal-axis wet buffing will be described by using the same reference numerals for representing the same parts as those mentioned above.

That is, in this step of preliminary finishing longitudinal-axis wet buffing, a design surface 2 is polished by the lower end surface of a buff 13 attached to the lower end portion of a rotation shaft 12 hung down onto the design surface 2, but electrolysis working as mentioned above is not performed.

Also in this step of preliminary finishing longitudinal-axis wet buffing, a rotary table 11 is driven at a low speed in the same manner as that mentioned above, and the rotating direction of the rotary table 11 is reversed at desired time intervals (for example, 30 seconds) also in the same manner as that mentioned above.

The buff 13 is formed of an extremely soft material in comparison with a conventional buff, which is, in this embodiment, a continuous bubble sponge material having a large expansion ratio also in the same manner as that mentioned above.

Also in the same manner as that mentioned above, while the urging force of the buff 13 against the design surface 2 is small, for example, not more than 1 kgf/cm², the lower end surface of the buff 13 can keep contacting with the design surface 2 in spite of the presence of the uneven portions of the design surface 2 as shown in FIG. 4.

Also in the same manner, the rotation shaft 12 supporting the buff 13 in its lower end portion is to be driven, for example, at the rotation speed 300 rpm so that the rotation speed of the buff 13 is made extremely low) about not more than 10 m/sec (for example, 4 m/sec), and the rotation shaft 12 is held by an elevator 15, and when the rotating direction of the rotary table 11 is turned to the opposite direction, the buff 13 is moved upward away from the design surface 2 by the elevator 15, and the rotating direction of the rotation shaft 12 is also turned to the opposite direction at the same time so as to be driven to rotate in the direction coincident with the rotating direction of the rotary table 11.

An abrasive liquid having the same components as that mentioned above, and including alumina of the particle diameter 10 μm as abrasive is supplied to the contact portion between the buff 13 and the design surface 2 through the shaft hole 12a of the rotation shaft 12 and an abrasive liquid nozzle 16, so that wet buffing is performed.

The reason why such a step of preliminary finishing longitudinal-axis wet buffing is performed following the above-mentioned step of preliminary finishing transversal-axis wet buffing is that polishing traces caused by the step of preliminary finishing transversal-axis wet buffing are erased by polishing in the direction perpendicular thereto in this step of preliminary finishing longitudinal-axis wet buffing.

Further, in accordance with this step of preliminary finishing longitudinal-axis wet buffing, since the buff 13 is soft and rotated at a low rotation speed, the buff 13 can follow the uneven portions of the design surface 2 so well that the buff 13 can polish the design surface 2 surely in spite of the presence of the unevenness, so that it is advantageously possible to effectively eliminate foreign matters put directly under the buff 13 by the rising and sinking operation of the buff 13 at the time of reversing the rotating direction of the rotary table 11, and obtain a superior finished surface surely.

Next a step of mirror finishing transversal-axis wet buffing (corresponding to the first polishing step according to the present invention) as a step of mirror finishing (corresponding to the surface finishing step according to the present invention) is performed (with reference to FIGS. 5 to 7) on the vehicle wheel blank 1 in which such a step of preliminary finishing longitudinal-axis wet buffing has been completed.

Since this step of mirror finishing transversal-axis wet buffing is performed almost in the same manner as the step of preliminary finishing transversal-axis wet buffing except for the abrasive liquid, the same means and steps as those mentioned above are omitted on description, and only terms relating to the different points will be described.

That is, the abrasive liquid used in the step of mirror finishing transversal-axis wet buffing in this embodiment is a solution including alumina of the particle diameter 1 μm, and a surface active agent added thereto.

The reason why this step of mirror finishing transversal-axis wet buffing is performed following the step of preliminary finishing longitudinal-axis wet buffing is that polishing traces caused by the step of preliminary finishing longitudinal-axis wet buffing using abrasive of the particle diameter 10 μm can be erased to increase brightness, and the reason why such a transversal-axis wet buffing is performed is that the polishing force of buffs 23a and 23b are so large in the transversal-axis wet buffing that the polishing traces can be erased effectively even by wet buffing with abrasive of minute particle diameter, so that it is possible to obtain an extremely superior bright finished surface.

From the same reason as the above-mentioned step of preliminary finishing transversal-axis wet buffing, in this step of mirror finishing transversal-axis wet buffing, it is possible to make a linear contact and a large polishing force caused by fresh abrasive supplied easily, it is possible to polish the inside of the design surface 2, and it is further possible to directly spray an abrasive liquid to portions in which the buffs 23a and 23b have flexibility, so that it is advantageously possible to cool the buffs 23a and 23b surely.

After that, a step of mirror finishing longitudinal-axis wet buffing (corresponding to a second polishing step according to the present invention) is performed on the vehicle wheel blank 1 (with reference to FIGS. 3 and 4), so that the design surface 2 of the vehicle wheel blank 1 is finished into a mirror-finished surface finally.

Since this step of mirror finishing longitudinal-axis wet buffing is performed almost in the same manner as the above-mentioned step of preliminary finishing longitudinal-axis wet buffing except for the abrasive liquid, the same means and steps as those mentioned above are omitted on description, and only terms relating to the different points will be described.

That is, the abrasive liquid used in the step of mirror finishing longitudinal-axis wet buffing in this embodiment is a solution including alumina of the particle diameter 1 μm, which is the same as the abrasive liquid used in the above-mentioned step of mirror finishing transversal-axis wet buffing.

The reason why this step of mirror finishing longitudinal-axis wet buffing is performed following the step of mirror finishing transversal-axis wet buffing is that minute polishing traces caused by the step of mirror finishing transversal-axis wet buffing having a large polishing force can be erased by polishing in the direction intersecting the polishing traces by use of the same abrasive liquid including abrasive of the same particle diameter.

In comparison with a conventional product formed by mold, the finished surface of the design surface 2 obtained thus has a mirror-finished state equivalent to a mirror-finished product finished by skilled worker's hand.

In accordance with this step of mirror finishing longitudinal-axis wet buffing, since the buff 13 is soft and rotated at a low rotation speed in the same manner as that in the step of preliminary finishing longitudinal-axis wet buffing, the buff 13 can follow the uneven portions of the design surface 2 so well that the buff 13 can polish the design surface 2 surely in spite of the presence of the uneven portions, so that it is advantageously possible to effectively eliminate foreign matters put directly under the buff 13 by the rising and sinking operation of the buff 13 at the time the rotary table 11 turns its direction, and obtain an extremely superior finished surface surely.

A step of forming a coating as follows is performed onto the design surface 2 of the vehicle wheel blank 1 finished thus into a mirror-finished bright surface so as to form a transparent coating 31 as shown in FIG. 9.

In this embodiment, a coating composite used in the step of forming coating is an inorganic-organic composite paint of a silica-silane group and is composed mainly of inorganic material. The coating composite includes a dispersion liquid, acrylic resin and a surface active agent.

The dispersion liquid of this coating composite consists of colloid silica which is 20 to 60 percent by weight, and partial hydrolysis condensation product of organo-alkoxy-silane which is 20 to 60 percent by weight, the percentage being relative to the total quantity of a solid component of the coating composite.

The acrylic resin of this coating composite consists of polymer or copolymer of unsaturated ethylenic monomer which is 5 to 40 percent by weight, and solvent for which glycol derivative is indispensable and which is 150 to 550 percent by weight, and this solvent is low level aliphatic alcohol such as methanol including glycol derivative not less than 10 percent by weight, the percentage being relative to the total quantity of a solid component of the coating composite.

The surface active agent of the coating composite is a nonionic surface active agent.

A viscosity gain agent,. a bubble erase agent, a dyestuff or the like may be added to this coating composite desirably in accordance with necessity.

The reasons why the respective components are added in accordance with the above-specified ranges will be described.

The colloid silica is contained by 20 to 60 percent by weight. If the lower limit would be exceeded, the hardness of the transparent coating 31 formed would be reduced, whereas if the upper limit would be exceeded, the collision-resistance factor thereof would be degraded.

The partial hydrolysis condensation product of organo-alkoxy-silane is contained by 20 to 60 percent by weight. If the lower limit would be exceeded, the contactability of the transparent coating 31 would be degraded, whereas if the upper limit would be exceeded, the collision-resistance factor thereof would be degraded.

The polymer or copolymer of unsaturated ethylenic monomer is contained by 5 to 40 percent by weight. If the lower limit would be exceeded, it would be difficult to form the coating layer over 20 μcm and cracks would be likely to be formed due to thermal contraction or the like. In this case, the collision-resistance factor and anti-corrosion factor would be degraded. If the upper limit would be exceeded, the hardness of the coating 31 would be reduced to deteriorate the anti-frictional surface damage and the anti-contamination property.

The glycol derivative is contained in the low level aliphatic alcohol over 10 percent by weight. The reason for this is that the aging stability of the coating would be enhanced, and it is possible to keep a good appearance of the coating 31 and to make it easy to produce it.

The reason why the nonionic surface activating agents are used is that the aging stability of the coating may be well maintained.

The transparent coating 31 is formed on the design surface 2 with such a coating composite by a normal step such as spraying or the like. In this case, the coating composite adheres to the design surface 2 so well that primary treatment is not necessary if the design surface 2 is kept cleaned.

After the coating composite is given onto the design surface 2, the coating composite is heated to 100 to 200 in for about 10 to 20 minutes by a furnace to be dried and hardened, thereby obtaining a transparent coating 31.

The transparent coating 31 formed thus on the design surface 2 has not only superior transparency and adhesion to the design surface 2, but also high impact resistance and crack resistance. Therefore, even in the case where the transparent coating 31 on the design surface 2 of the vehicle wheel blank 1 in this embodiment is formed thinner than a conventional coating, for example, to about 10 μm, the transparent coating 31 can protect the design surface 2 from being injured.

This means that the thickness of the transparent coating 31 can be made so thin that the transparency of the transparent coating 31 becomes superior to conventional one, but there is no fear that the injure and finishing unevenness of the design surface 2 itself is substantially recognized by eyes since the design surface 2 is finished into an extremely superior mirror-finished surface as has been described above.

Further, since this transparent coating 31 has superior corrosion resistance and adhesion, corrosion is hardly generated in the design surface 2, so that its initial brightness can be maintained for a long time, and since the transparent coating 31 has superior stain-proof, it is easy and possible to eliminate stain adhering to the surface of the transparent coating 31, so that it is advantageously possible to maintain the brightness easily.

Therefore, in the vehicle wheel blank 1 according to this embodiment, the design surface 2 is finished to an extremely superior finished surface, and the transparent coating 31 formed on the design surface 2 has extremely superior transparency, so that the superior brightness of the design surface 2 can be recognized through the transparent coating 31 obviously by eyes, so that it is possible to make the appearance of the vehicle wheel blank 1 extremely superior.

The transparent coating 31 may be formed with either organic paint or inorganic paint. In the latter case, it is advantageous to prevent the generation of a so-called string rust on an automotive aluminum wheel.

As has been described, according to this embodiment, an aluminum-alloy cast product is used as a subject to be worked, and both the steps of preliminary and mirror finishing are performed thereon by wet buffing, so that minute pores or pin-holes opening to the surface of the cast product are made closed and small with the contact friction of buffs, so that it is advantageously possible to reduce defects on the finished surface of the cast product to obtain a superior finished surface.

The present invention can be applied to a vehicle wheel of a forged product as well as a cast product, and the material of a subject to be worked is not limited to an aluminum alloy, but any other metal material such as a copper alloy may be used so long as the metal material may be a subject to be mirror-finished.

As has been described, according to the invention stated in claim 1, a soft buff is used, so that the surface of a product formed by mold can be polished with the buff following the unevenness of the surface.

Further, since such buffing is a so-called wet buffing with an abrasive liquid, the rotation speed of the buff is lower than that in dry buffing with no abrasive liquid, so that the buff can transform correspondingly to the uneven portions of the surface well enough to follow the uneven portions of the surface, so that the surface can be polished surely in spite of the presence of the unevenness.

Further, since a buffing device having a transversal rotation axis contacts the circumferential surface of the buff with the molded product, the contact area between the buff and the molded product is linear and therefore small, so that it is possible to supply fresh abrasive through the abrasive liquid to this contact area surely, and it is possible to exclude foreign matters such as buffing dusts efficiently to make it possible to obtain a superior finished surface by buffing the surface of a molded product with a machine.

Therefore, since finishing a design surface as a substrate for a transparent coating is improved, the conventional contradictory situation in which the appearance of the design surface is lowered can be solved even under the improvement of the transparency of the transparent coating, so that it is possible to improve the appearance of a vehicle wheel.

Further, since a buffing device having a transversal rotation axis and a soft buff is used, the foregoing contact area is small, so that it is possible to supply an abrasive liquid to the vicinity of this contact area surely. Accordingly, as an additional advantage, it is possible to effectively cool heat generation of the buff per se due to elastic transformation in the contact area to thereby prolong the lifetime of the soft buff.

Another embodiment of the invention will be described with FIGS. 10 and 11. This embodiment is a wet buffing method for polishing the surface of a workpiece in a manner that the surface of the workpiece is brought into contact with a buff which is rotated supplying an abrasive liquid containing abrasive to the surface of the workpiece.

In this type of the wet buffing method, the abrasive is supplied to the buff by use of the abrasive liquid containing the abrasive therein.

Since the polishing operation with the buff is performed in a contact area between the buff and the workpiece, the abrasive liquid containing the abrasive is supplied so as to reach the central part of the contact area.

The abrasive thus supplied to the contact area is worn through the polishing action, and must be exhausted from the contact area in order to improve the efficiency of the polishing action. If a great deal of the worn abrasive is left in the contact area, metal powder produced by the polishing action attach to the worn abrasive, possibly causing minute injuries on the surface of the workpiece or unevenness of the finishing.

In the conventional technique using this type of the wed buffing method, to prevent the deterioration of the surface of the workpiece, the buff is replaced with a new one at proper time intervals.

The buff replacing work is troublesome. In addition, the attainment of the good finished surface of the workpiece requires a frequent replacement work. This results in reducing the wed buffing efficiency.

This embodiment, which is presented for solving the above-mentioned problems, realizes a high efficiency operation of the wed buffing even in a case where the excellent finishing of the surface of the workpiece is required, with an infrequent buff replacement.

To achieve the above object, there is provided a wet buffing method for polishing the surface of a workpiece in a manner that the surface thereof is brought into contact with the buff mounted to a rotation shaft while supplying an abrasive liquid containing abrasive to the surface of the workpiece, wherein during the step of polishing the surface of the workpiece with the buff, the buff is moved so as to increase a gap between the buff and the workpiece surface, and then is moved so as to decrease the gap therebetween till the buff comes in contact with the workpiece surface, and during the polishing step, the supply of the abrasive liquid to the surface of the workpiece is continued.

In the wet buffing method of the invention, the abrasive liquid is supplied to the gap between the buff and the surface of the workpiece in a state that as the result of the movement of the buff toward the surface of he workpiece, the urging force of the buff against the workpiece is reduced. In this state, the worn abrasive, metal powder, and the like, which have thus far been held between the buff and the workpiece, are released from the state of being tightly held, so that those are acceleratedly exhausted by the abrasive liquid.

The residual materials, such as worn abrasive, in the gap between the buff and the surface of the workpiece is reduced, so that the minute injuries on the surface of the workpiece and the unevenness of the finishing are reduced to a minimum.

Accordingly, the troublesome buff replacing work is infrequently required. The efficient wet buffing can be carried out even where the high quality surface polishing is required.

Hereunder, the preferred embodiment of the present invention will be described with reference to FIGS. 10 and 11. In this embodiment, the technical idea of the present invention is applied to vehicle wheels which are cast products of an aluminum alloy. In this specification, the design surface means a surface which faces outward from a vehicle body and forms a part of appearance of a vehicle, when the wheel is mounted on the vehicle. Like reference numerals are used for designating like or equivalent portions throughout the drawings of FIGS. 3 to 9.

First, a vehicle wheel blank 1 as a workpiece is a cast product obtained by pouring a molten aluminum (for example, AC4CH) alloy into a mold having a predetermined shape.

The vehicle wheel blank 1 thus obtained is taken out of the mold and machined in its predetermined portion in a step of machining so as to be formed into a predetermined accurate shape as the vehicle wheel blank 1.

In the vehicle wheel blank 1, a number of uneven portions are formed by spoke portions 3, through holes 4, hub portions 5, rim portions 6 and so on from the request of design (with reference to FIGS. 3 and 4).

The design surface 2 (corresponding to the surface of the invention) of the vehicle wheel blank 1 is subjected to the steps of a rough fining and a half finishing. Then, the wet buffing method to be given hereinafter is performed on the design surface 2 to be finished into a bright finished surface at last.

The wet buffing method of the present embodiment will be described (with reference to FIGS. 3 and 4).

That is, in this wet buffing method, the vehicle wheel blank 1 is fixed on a rotary table 11 so that its center is made to coincident with the rotation center O of the rotary table 11 and the design surface 2 is fixed to its upward and horizontal attitude. Then, the design surface 2 is polished by the lower end surface 13a of a buff 13 attached to the lower end portion of a rotation shaft 12 hung down.

The rotary table 11 is driven to rotate at a low speed at the time of working and the portion to be worked by the buff 13 is displaced circumferentially sequentially. In this embodiment, the rotating direction of the rotary table 11 is reversed at desired time intervals (for example, 30 seconds). The buff 13 is formed of an extremely soft material in comparison with a conventional buff, for example, in this embodiment, a continuous bubble sponge material having a large expansion ratio (for example, 3 to 5). The buff 13 is not limited to a sponge material, but any other material may be used as the buff 13 so long as it is as soft as the sponge material.

Therefore, the buff 13 can keep contacting with the design surface 2 surely in spite of the presence of the uneven portions of the design surface 2 as shown in FIG. 4.

The urging force of the buff 13 against the design surface 2 is small, for example, not larger than 1 kg/cm².

The rotation shaft 12 supporting the buff 13 in its lower end portion is held vertically movably by an elevator 15 constituted by an oil pressure cylinder and so on. The rotation shaft 12 is driven, for example, at a rotation speed of 300 rpm so that the rotation speed of the buff 13 is made to be extremely low not higher than 10 m/sec, for example, 4 m/sec in this embodiment.

This is because the above-mentioned soft material constituting the buff can be deformed surely following the uneven portions of the design surface 2, while keeping its contact state with the uneven portions.

This rotation shaft 12 is driven to rotate in the direction which is coincident with the rotating direction of the rotary table 11, and if the rotating direction of the rotary table 11 is reversed to the opposite direction, the rotation shaft 12 is pulled up by the elevator 15 so that the buff 13 moves upward away from the design surface 2, and the rotation shaft 12 is driven to rotate in the opposite direction at the same time.

Accordingly, even if the rotating direction of the rotary table 11 is reversed, the respective rotating directions of the rotary table 11 and the buff 13 are always kept to be coincident with each other.

A shaft hole 12a is formed in the rotation shaft 12, so that an abrasive liquid containing abrasive added thereto is supplied to the center portion of the buff 13 through this shaft hole 12a.

This abrasive liquid is an aqueous solution containing alumina of 10 μm in particle diameter as abrasive in this embodiment, and containing also a surface active agent added thereto.

Further, an abrasive liquid nozzle 16 for supplying the same abrasive liquid as mentioned above to a contact portion or area between the buff 13 and the design surface 2 is provided to supply fresh abrasive to the contact area of the buff 13 and the design surface 2 from the outside of the contact area surely as shown by a phantom line in FIG. 3.

The abrasive liquid is supplied at approximately 100 cc/min from the shaft hole 12a of the rotation shaft 12 and the abrasive liquid nozzle 16.

The supply of the abrasive liquid from this abrasive liquid nozzle 16 is continued even when the rotating direction of the rotary table 11 is reversed.

In this embodiment, the rotation of the rotary table 11, the rotation of the buff 13, the movement of the rotation shaft 12, and the supply of the abrasive liquid are timed as illustrated in the timing chart of FIG. 10.

In FIG. 10, the abscissa represents a time axis. At time t0 the rotary table 11 is rotating clockwise (CW). The CW rotation of the rotary table 11 stops at time t1. During a time period from time t0 to t1, the rotation shaft 12 with the buff 13 fixed thereto keeps a state that the buff 13 is urged against the design surface 2 by a predetermined urging force at a descending position as indicated by a solid line in FIG. 4 (in FIG. 10, it is denoted as "Contact"). The buff 13 is being rotated counterclockwise at a predetermined speed by the rotation shaft 12. During this period, the abrasive liquid is supplied to the contact area of the lower end surface of the buff 13 and the design surface 2 by way of the shaft hole 12a of the rotation shaft 12 and the abrasive liquid nozzle 16.

During the period from time t0 to t1, the design surface is polished by the abrasive contained in the abrasive liquid, in the contact area of the buff 13 and the design surface 2.

At time t1, the rotary table 11 and the buff 13 is revered in the rotating direction, so that the table and buff are both stopped in rotation. The elevator 15 lifts the rotation shaft 12. The lower end surface of the buff 13 is moved away from the design surface 2 (to each the ascending position as indicated by the phantom line in FIG. 11 and denoted as "Separate" in FIG. 10). Also in this state, the supply of the abrasive liquid is carried out as in the previous case.

The abrasive liquid is thus supplied to the portion of the design surface where the buff 13 was in contact with the design surface 2, in a state that the buff 13 is separated from the design surface 2. Accordingly, worn abrasive, metal powder, and the like, which have been held in the contact area till time t1, is washed out by the abrasive liquid.

This state is continued for a short time from time t1 to t2, for example, about 5 seconds.

At time t2 the counterclockwise (CCW) turn of the rotary table 11 starts. The elevator 15 descends. With descending of the elevator 15, the rotation shaft 12 starts to turn counterclockwise in a state that, as already stated, at the descending position it urges the buff 13, by a predetermined urging force, against the design surface 2. Since the abrasive liquid is supplied as in the previous case, the design surface is polished in the contact area also during the period of time from time t2 to t3, as in the case of the CW turn.

Since the worn abrasive, metal powder and the like, which were caused during the time period from time t1 to t2, have been exhausted, fresh abrasive are readily dispersed over the contact area, ensuring a good polishing operation.

The time period from time t2 to t3 is about 30 seconds, for example. The CW turn of the rotary table 11 and the like also continues for the substantially equal time period.

At time t3, the rotating direction of the rotary table 11 and the buff 13 is revered to the CW rotating direction, and are stopped in their rotation.

The lower end surface of the buff 13 and the design surface 2 are separated,from each other by the elevator 15 and then by the rotation shaft 12, and the supply of the abrasive liquid continues, as in the operation for the time period from t1 to t2.

The worn abrasive, metal powder and the like, which have been arrested in the contact region till time t3, are washed away by the abrasive liquid, and from time t4 the rotary table 11 and the buff 13 start to turn in the CW direction. The subsequent operation resembling that from time t1 to t2 progresses.

The rotation, stop, and reverse of the rotary table 11 and the buff 13 are repeated at proper time intervals, to polish the design surface 2 into a bright finished surface.

In this embodiment, with the reversal of the rotating direction of the rotary table 11 and the buff 13, the buff 13 is lifted from the design surface 2. In this separated state, the abrasive liquid is supplied to the portion on the design surface 2 where the buff 13 was in contact with the design surface 2. Accordingly, the foreign materials, such as the worn abrasive, which have been arrested in the contact region during the polishing operation, are surely removed.

The time that the life of the buff 13 continues is elongated. There is no need of frequently replacing the buff 13 with a new one. The polishing operation can be efficiently performed in such a case where the wet buffing method is used for obtaining a good finished surface.

In the above-mentioned embodiment, the foreign materials, such as worn abrasive, are removed utilizing the reversal of the turn of the rotary table 11 and the like. The same purpose can be attained by an alternative as shown in FIG. 11.

The timing chart shown in FIG. 11 is equivalent to that of FIG. 10, which additionally includes chances P1 and P2 to remove worn abrasive and the like. Description will be given hereunder on only the abrasive removal chances.

The operation described in the timing chart of FIG. 11 for the wet buffing method has the advantage over the operation described in the timing chart already described where the worn-abrasive removal chances are present between time points t1 and t2, and between time points t3 and t4, in that the range of the vertical movement of the rotation shaft 12 is narrow, and the time for the vertical movement is short.

With provision of the chances P1 and P2, as seen from FIG. 11, the rotation shaft 12 is slightly lifted for a short time during the CCW turn of the rotary table 11 and the buff 13, thereby to move the buff 13 away from the design surface 2 to such an extent that the buff 13 is not detached from the design surface 2, viz., to weaken the urging force of the buff against the design surface.

As a result, the force to catch and hold the worn abrasive and the like between the buff 13 and the design surface 2 is lessened, thereby to slightly promote the action of the abrasive liquid to wash away the worn abrasive out of the contact area and to supply fresh abrasive thereto. The time that the life of the buff 13 continues is elongated. With an infrequent replacement of the buff 13, the polishing operation can be efficiently performed in such a case where the wet buffing method is used for obtaining a good finished surface.

It is preferable that the chances P1 and P2 to remove the worn abrasive and to supply the abrasive afresh recur at short time intervals, e.g., about 5 seconds, so as to allow the successive vertical movement of the buff 13 during the polishing operation. Such a short time recurrence of the chances can keep the abrasive, which acts to polish the design surface between the buff 13 and the design surface 2, in good conditions.

In this case, if the buff 13 is moved so as to disengage from the design surface 2, fresh abrasive efficiently replaces the old abrasive present between the buff 13 and the design surface 2, ensuring a good polishing work by the abrasive. Accordingly, an excellent polished surface can be surely obtained by the polishing work for a relatively short time.

The above-mentioned embodiment employs the vertical shaft of the rotation shaft 12 for rotating the buff, but may employ the horizontal shaft, if required.

As described above, the abrasive liquid is supplied to the portion between the buff and the workpiece in a state that the urging force of the buff against the workpiece is 32 reduced by the vertical movement of the buff. Under the reduced urging force, the force of catching and holding the worn abrasive, metal powder and the like between the buff and the workpiece is weakened, thereby promoting the removal of the worn abrasive. As a result, a small amount of the worn abrasive and the like is left between the buff and the workpiece, thereby minimizing the minute injuries on the surface of the workpiece and the unevenness of the finishing.

With an infrequent replacement of the buff 13, the polishing operation can be efficiently performed in such a case where the wet buffing method is used for obtaining a good finished surface.

Still another embodiment of the invention will be described with reference to in FIGS. 12 through 15. This embodiment is directed to an electrolysis composite-type buffing method for simultaneously performing an electrolysis working and a buffing working on the surface of a workpiece.

There has been known a called electrolysis buffing method for simultaneously performing an electrolysis working and a buffing working on the surface of a workpiece, in order to effectively finish the surface thereof to obtain a good finished surface (as disclosed in Japanese Patent Publication No. Sho. 60-48289).

In this type of the electrolysis composite-type buffing, a passive state oxide film is formed on the surface of a workpiece by the electrolysis action, and the irregular surface of the oxide film is smoothed by the buffing. Accordingly, the electrolysis composite-type buffing can efficiently finish the surface of the workpiece into a good finished surface.

Also in the electrolysis composite-type buffing, presence of abrasive in the contact area between the buff and the surface of the workpiece is essential to remove the irregularity on the workpiece surface (or to work the workpiece). The abrasive is worn through the working of the workpiece.

The buff with abrasive bonded thereto or coated with abrasive is used in the electrolysis composite-type buffing. The buff effectively works the workpiece in the initial stage of using the buff, but the working efficiency by the buff is deteriorated after it is used to a certain extent. To avoid this, the buffer is replaced with a new one to render the abrasive afresh.

Where the electrolysis composite-type buffing is used, the electrolysis working is operable continuously for a long time, but the buffing is interruptive because the abrasive must be refreshed for a relatively short time interval. To refresh the abrasive, the working must be stopped. Accordingly, the working efficiency by the electrolysis composite-type buffing is not good.

For the above circumstances, the present invention is to provide an electrolysis composite-type buffing method in which an electrolysis composite-type buffing is continuously operable for a long time, thereby improving the efficiency of the electrolysis composite-type buffing.

To achieve the above object, there is provided an electrolysis composite-type buffing method in which a buff with an electrode, which is fastened to one end of the rotating shaft, is set on the surface of a workpiece fixed to a work table, for the electrolysis working, electrolyte is supplied to the portion between the electrode and the workpiece, and electrolysis current is fed thereto, and for the buffing, the buffer is rotated by the rotation shaft, the improvement wherein abrasive is added to the electrolyte supplied.

In the electrolysis composite-type buffing method thus constructed, a buff with an electrode is provided, and electrolyte is supplied to the portion between the electrode and the workpiece, and abrasive is added to the electrolyte supplied to between the electrode and the workpiece. Accordingly, the abrasive used for the buffing is continuously supplied to the contact area between the buff and the surface of the workpiece.

With the supply of the electrolyte, the abrasive is continuously refreshed, so that the interruption of the buffing infrequently recurs.

Accordingly, in the electrolysis composite-type buffing method, the buffing can be continued for a long time using the fresh abrasive continuously supplied. A good finished face can be obtained in a continuous manner, thus leading to the improvement of the efficient operation of the electrolysis composite-type buffing.

Hereunder, the preferred embodiment of the present invention will be described with reference to FIGS. 10 and 11. In this embodiment, the technical idea of the present invention is applied to vehicle wheels which are cast products of an aluminum alloy. In this specification, the design surface means a surface which faces outward from a vehicle body and forms a part of appearance of a vehicle, when the wheel is mounted on the vehicle.

First, a vehicle wheel blank 1 as a workpiece is a cast product obtained by pouring a molten aluminum (for example, AC4CH) alloy into a mold having a predetermined shape.

The vehicle wheel blank 1 thus obtained is taken out of the mold and machined in its predetermined portion in a step of machining so as to be formed into a predetermined accurate shape as the vehicle wheel blank 1.

In the vehicle wheel blank 1, a number of uneven portions are formed by spoke portions 3, through holes 4, hub portions 5, rim portions 6 and so on from the request of design (with reference to FIGS. 3 and 4).

The design surface 2 (corresponding to the surface of the invention) of the vehicle wheel blank 1 is subjected to the steps of a rough fining and a half finishing. Then, the wet buffing method to be given hereinafter is performed on the design surface 2 to be finished into a bright finished surface at last.

The electrolysis composite-type buffing method of the present embodiment will be described (with reference to FIGS. 3 and 4).

That is, in this electrolysis composite-type buffing method, the vehicle wheel blank 1 is fixed on a rotary table 11 (corresponding to "work table" in this specification) so that its center is made to coincident with the rotation center O of the rotary table 11 and the design surface 2 is fixed to its upward and horizontal attitude. Then, the design surface 2 is polished by the lower end surface of a buff 13 attached to an end plate 12b of the lower end portion of a rotation shaft 12 hung down.

The rotary table 11 is driven to rotate at a low speed at the time of working and the portion of the vehicle wheel blank 1 as a workpiece, which is to be worked by the buff 13, is displaced circumferentially sequentially. In this embodiment, the rotating direction of the rotary table 11 is reversed at desired time intervals (for example, 30 seconds). The entire of the buff 13 is formed substantially of a continuous bubble sponge material. In the buff 13, an electrode 14 lies in a cross direction along the center line of the cross section of the buff and in parallel with the end plate 12b (with reference to FIG. 12).

The sponge material constituting the buff 13 has a large expansion ratio (for example, 3 to 5), and is an extremely soft material in comparison with a conventional buff. The buff 13 is not limited to a sponge material, but any other material may be used as the buff 13 so long as it is as soft as the sponge material.

The buff 13 in this embodiment is formed of a substrate part 13a located on upper side of the electrode 14 and a tool part 13b on the lower side of the electrode 14. The tool part 13b functions as a polishing part of the buff, and the substrate part 13a supports the electrode 14 and the tool part 13b, both being located on the lower side of the substrate part.

In this embodiment, the substrate part 13a and the tool part 13b are both formed of the same kind of materials, but may be formed of different kinds of materials.

As described above, the substrate part 13a and the tool part 13b of the buff 13 are formed of extremely soft material. Accordingly, the buff 13 is flexible, and the lower end surface of the buff 13 can keep contacting with the design surface 2 surely in spite of the presence of the uneven portions of the design surface 2.

The electrode 14 of the buff 13 is formed of a mesh as a flexible sheet-like member, and has the same shape as that, or a planar shape, of the buff 13.

Use of the substrate part 13a made of sponge material on the electrode 14 provides unique and beneficial effects in the electrolysis composite-type buffing method as will apparent from the description following the description of the buffing method of the embodiment.

The buff 13 thus constructed is mounted on an insulating plate 18, which is also mounted on the lower side of the end plate 12b. A DC power supply 17 is inserted between the electrode 14 and the vehicle wheel blank 1. The electrode 14 is connected to the positive polarity of the DC power supply 17, and the design surface 2 of the vehicle wheel blank 1 is connected to the negative polarity.

The rotation shaft 12 supporting the buff 13 in its lower end portion is held vertically movably by an elevator 15 constituted by an oil pressure cylinder and so on. The urging force of the buff 13 against the design surface 2 is small, for example, not larger than 1 kg/cm².

The rotation shaft 12 is driven, for example, at a rotation speed of 300 rpm so that the rotation speed of the buff 13 is made to be extremely low not higher than 10 m/sec, for example, 4 m/sec in this embodiment.

This is because the above-mentioned soft material constituting the tool part 13b of the buff can be deformed surely following the uneven portions of the design surface 2, while keeping its contact state with the uneven portions.

This rotation shaft 12 is driven to rotate in the direction which is coincident with the rotating direction of the rotary table 11, and if the rotating direction of the rotary table 11 is reversed to the opposite direction, the rotation shaft 12 is pulled up by the elevator 15 so that the buff 13 moves upward away from the design surface 2, and the rotation shaft 12 is driven to rotate in the opposite direction at the same time.

Accordingly, even if the rotating direction of the rotary table 11 is reversed, the respective rotating directions of the rotary table 11 and the buff 13 are always kept to be coincident with each other.

A shaft hole 12a is formed in the rotation shaft 12. An electrolyte is supplied to the lower end surface of the buff 13, through the shaft hole 12a, the substrate part 13a and the tool part 13b (see an arrow in FIG. 12).

An electrolyte nozzle 16 for supplying the electrolyte to a contact portion or area between the buff 13 and the design surface 2 is provided as indicated by a phantom line 3.

The electrolyte supplied from the shaft hole 12a and the electrolyte nozzle 16 penetrates the sponge material of the tool part 13b between the electrode 14 and the design surface 2 to form an electrolysis circuit including the DC power supply 17. With the current flowing throughout the formed electrolysis circuit, an electrolytic action is performed on the surface of the design surface 2.

The electrolyte in this embodiment is a surface-active contained solution including sodium nitrate for the electrolytic action.

Since the electrolyte is supplied to the contact area between the buff 13 and the design surface 2 as mentioned above, abrasive for the buffing by the buff 13 is also added to the electrolyte.

The abrasive contained in the electrolyte is an alumina of the particle diameter 10 μm. The electrolyte is continuously supplied to the contact area between the buff 13 and the design surface 2, so that the electrolytic action and the supply of fresh abrasive can be performed simultaneously.

The electrolyte of about 100 cc/min or more, for example, is supplied through the shaft hole 12a of the rotation shaft 12 and the An electrolyte nozzle 16.

In the electrolysis composite-type buffing method, the electrolysis working and the buffing with abrasive are concurrently performed in a composite manner, that is, a called electrolysis composite-type buffing is continuously performed. Accordingly, the working efficiency is superior to that in ordinary buffing. Further, the buff replacement attendant with the stop of the working is infrequent, improving the efficiency of the working.

The supply of the electrolyte from the electrolyte nozzle 16 is continued also when the rotary table 11 is turned reversely. At the time of the reverse turn of the rotary table 11 and of ascending of the buff 13, the electrolyte is sprayed toward the portion of the design surface, which has been right under the buff 13. The sprayed electrolyte washes away buffing dusts and other foreign materials, which have thus far been left in the contact portion between the buff 13 and the design surface 2.

In a case where the workpiece is made of an aluminum alloy, silicon crystalline appears on the surface of the workpiece as the result of the electrolysis working. In this embodiment, the silicon crystalline are removed by the buffing since the wet buffing is performed concurrently with the electrolysis working.

The provision of the sponge substrate part 13a on the lower side of the electrode 14 brings about the following beneficial effects.

Where the electrolysis working and the buffing are combined as in the embodiment, the distance between the electrode 14 and the design surface 2 is an important control factor greatly influencing the working efficiency and the finished surface in the electrolysis working.

In the buffing, selection of a proper urging force of the lower side of the buff 13 against the design surface 2 is also an important factor in obtaining a good finished surface. In the molded product of which the surface to be finished is uneven, such as the vehicle wheel blank 1, the lower side of the buff 13 must be kept contacting with the surface in spite of the uneven surface. To cope with this, the buff 13 is made of a soft material. Accordingly, the height of the electrode 14 as measured from the design surface 2 greatly varies.

The requirement for the buff material in the buffing and the requirement for the height of the electrode 14 in the electrolysis working are contradictive. The contradiction arises from the intrinsic nature of the electrolysis composite-type buffing method.

The reason why the sponge substrate part 13a is provided on the electrode 14 in the buff 13 is to lessen the requirement contradiction between the electrolysis working and the buffing, and to form a good finished surface.

The functions of the substrate part 13a will be described in detail with reference to FIGS. 13A and 13B.

FIG. 13A is an explanatory diagram showing how an electrolysis composite-type buffing is performed on the surface 22 of a planar workpiece 21.

Since a predetermined urging force is made to act on the buff 13, the lower side of the buff 13 is raised distance m from a free state position P as indicated by a phantom line. With the raising motion, the gap between the surface 22 of the workpiece 21 and the end plate 12b is decreased.

Accordingly, if the electrode 14 is fastened to the end plate 12b, the gap between the surface 22 of the workpiece 21 and the end plate 12b is varied by the distance m. This results in a great variation of the electrolysis working condition.

In the buff 13, the electrode 14 is provided retractively with respective to the axial direction of the rotation shaft 12 in a state that the substrate part 13a of sponge material is interposed between the electrode and the end plate of the rotation shaft. Accordingly, the electrode is pushed up distance n from the free state position P of the electrode, and a quantity of the displacement of the electrode 14 from the surface 22 of the workpiece 21 is reduced by distance m-n.

The current flowing from the electrode 14 to the surface 22 concentrates at the portion where the distance between them is the shortest. However, the current concentration is lessened since the displacement quantity of the electrode 14 is reduced as stated above. Accordingly, the electrolysis working is performed over the surface with a satisfactory uniformity, to obtain a uniformly finished surface.

With the above-mentioned construction of the buff 13, properly selecting the urging force of the buff 13 reduces a frequency of the adjustment of the electrolysis conditions, thus providing a good finished surface.

When the electrolysis composite-type buffing is applied to such a planar surface 22, the electrode 14 may be made of material of a high rigidity, not the flexible material.

FIG. 13B is an explanatory diagram showing how an electrolysis composite-type buffing is performed on the uneven surface 24 of a planar workpiece 23 such as a molded product. In the electrolysis composite-type buffing, the current flowing from the electrode 14 to the surface 24 concentrates at the portion where the distance between them is the shortest. In the construction as shown, if the electrode 14 is constructed with a mesh as a flexible sheet-like member, the useful effects as mentioned above can be attained although for the local areal portion of the surface.

When the buff 13 is positioned on the upwardly curved portion 24a of the surface 24 of the workpiece 23, the lower side of the buff 13 is pushed upward distance m corresponding to the height of the upwardly curved portion 24a. With this, the electrode 14 is locally raised distance n in the portion corresponding to the upwardly curved portion 24a.

In this case, if the electrode 14 is rigid, the distance between the electrode 14 and the upwardly curved portion 24a of the surface 24 of the workpiece 23 is distance m smaller than that between the electrode 14 and the other areal portion than the upwardly curved portion 24a. The current from the electrode 14 concentrates at the upwardly curved portion 24a, so that the upwardly curved portion 24a is excessively electrolyzed. The current density in the remaining portion of the surface 24 is low, so that the electrolytic action there is weak or unsatisfactory.

In the buff 13, the electrode 14 is also locally displaced distance n in the upward direction. In the portion of the electrode corresponding to the upwardly curved portion 24a, the distance or gap width between the electrode 14 and the surface 24 of the workpiece 23 is reduced only distance m-n. The difference of the current densities is smaller than in the previous case, so that the surface of the workpiece is relatively uniformly electrolyzed.

Accordingly, use of the buff 13 of this embodiment diminishes the nonuniform electrolysis working, thereby providing a good finished surface.

The thus useful buff having the substrate part 13a made of the sponge material may be made of any other suitable material and may have any other suitable structure.

A buff 31 structured as shown in FIG. 14 functions like the above-mentioned buff. In the description of this buff, like reference numerals in the above-mentioned embodiment handling the buff 13 are used for designating like or equivalent portions of the buff 31. Description will be given placing emphasis on only the difference of the buff 31 from the buff 13.

In the buff 31 as a modification of the buff 13, the substrate part 13a of the buff 13 is substituted by a ring-like cushion member 33 containing an air chamber 32 shaped like a ring. An electrode 14, formed of a mesh as a flexible sheet member, lies underneath the ring-like cushion member 33. The tool part 13b of the buff lies on the lower side of the electrode 14.

Also in the buff 31, the ring-like cushion member 33, like the buff 13, is entirely or locally deformed under the urging force or by the unevenness of the surface, because the cushion member 33 contains the air chamber 32. The deformation diminishes the variation of the gap width between the electrode 14 and the workpiece surface. Accordingly, a good finished surface can be secured for the same reasons as mentioned above.

While in the above-mentioned embodiments, the buff of the vertical shaft type in which the rotation shaft 12 is vertically disposed, it is evident that the buff of the horizontal shaft type in which the rotation shaft 12 is horizontally disposed as shown in FIG. 15.

The embodiment of the invention using the horizontal shaft type buff shown in FIG. 15 will be described.

In FIG. 15, a buff 35 is of the horizontal shaft type in which the rotation shaft 12 is disposed substantially parallel to the design surface 2. The buff 35 buffs the design surface 2 in a state that it is in contact with the design surface 2. The electrolyte is supplied through the electrolyte nozzle 16 as in the above-mentioned embodiments.

The buff 35 of a three-layer structure is formed of a substrate part 35a made of sponge material layered around the rotation shaft 12, a tubular electrode 36 layered around the substrate part 35a and coaxial with the rotation shaft 12, and a tool part 35b made of the sponge material, which is layered around the electrode 36.

A DC power supply 17 is inserted between the electrode 36 of the buff 35 and the design surface 2, thereby forming an electrolysis circuit.

The embodiment using the horizontal shaft type buff functions as the embodiments using the vertical shaft type buff, and has similar useful effects.

When the horizontal shaft type buff is used, the tool part 35b linearly contacts with the design surface 2 as the workpiece. With this linear contact, the electrolysis current is uniformly distributed, and the urging force of the buff against the design surface is intensified, so that the buffing force is strong. Accordingly, the electrolysis composite-type buffing is efficiently performed with the feed of a relatively small current.

In the electrolysis composite-type buffing method thus constructed, a buff with an electrode is provided, and electrolyte is supplied to the portion between the electrode and the workpiece, and abrasive is added to the electrolyte supplied to between the electrode and the workpiece. Accordingly, the abrasive used for the buffing is continuously supplied to the contact area between the buff and the surface of the workpiece.

With the supply of the electrolyte, the abrasive is continuously refreshed, so that the interruption of the buffing infrequently recurs.

Accordingly, in the electrolysis composite-type buffing method, the buffing can be continued for a long time using the fresh abrasive continuously supplied. A good finished face can be obtained in a continuous manner, thus leading to the improvement of the efficient operation of the electrolysis composite-type buffing.

Yet another embodiment of the present invention will be described. This embodiment is directed to a mirror-polished aluminum alloy blank of which surface is buffed into a mirror surface having the surface roughness of not larger than 1 μm in the center-line average roughness.

An extremely accuracy finishing work is required in order to polish the surface of a metal blank into a mirror surface having the surface roughness of not larger than 1 μm in the center-line average roughness. Accordingly, the blank to be mirror polished must be homogenous.

Before an aluminum alloy blank is finished into a mirror surface, a preliminary process is performed on the aluminum alloy blank in the following manner. The blank is subjected to a predetermined heat treatment, the hardness of the resultant blank is inspected, and the homogenous aluminum alloy blank to be mirror finished is obtained.

However, the mirror finishing work for the aluminum alloy blank made homogenous by the heat treatment frequently fails to form a mirror finished surface. The mirror finishing work of high working cost comes to nothing.

The inventor of the present Patent Application, who wrestled with this problem, discovered the fact that presence of the crystallized-out phase, such as the compound phase, crystallized out into spaces among dendrite arms in the surface region of the blank, greatly influences the formation of a mirror surface. On the basis of this discovery, the inventor reached the technical idea of the present invention that the limiting of the crystallized-out phase within a predetermined range provides an aluminum alloy blank which can reliably be finished into a mirror surface.

The embodiment of the present invention to be given hereinafter, which is based on the above-mentioned technical idea, remarkably reduces the occurrence of such an adverse situation that the mirror finishing work of high working cost is brought to nothing.

To this end, the present embodiment sets a mean value of the secondary dendrite arm spacings (referred to as DAS2) in the surface region of an aluminum alloy blank to be mirror finished, to a value of not larger than 40 μm. The metal structure of most industrial aluminum alloy blanks consists of the dendrite phase and the crystallized-out phase crystallized out into spaces among the dendrite arms. The crystallized-out phase has a large hardness than the dendrite. When a blank is buffed for the mirror finishing, the finishing working of the dendrite more efficiently progresses than that of the crystallized-out phase. As a result, the dendrite phases are curved downward to form depressions in the surface region as viewed in a cross section.

If the mean value of the DAS2 in the surface region of an aluminum alloy blank to be mirror finished is set to a value of not larger than 40 μm, the spaces among the crystallized-out phases of larger hardness are reduced. In the surface region of the blank of which the mean value of the DAS2 is so selected, an excessive working of the dendrite portions is controlled to a minimum. Accordingly, the surface of the dendrite portions is a little curved downward to form shallow depressions, so that a reliable mirror finishing is ensured.

Accordingly, such an unwanted situation that the mirror finishing work of high working cost is brought to a nothing, little occurs.

The present embodiment will be described in detail with reference to FIGS. 16 through 18, and then a specific example will be presented as a confirmation test.

In FIG. 16, reference numeral 101 designates an aluminum alloy blank to be mirror finished (referred to as a blank); numeral 102, dendrite cells; and 103, crystallized-out phases. The crystallized-out phases 103 are crystallized out to fill the spaces among the dendrite cells 102.

The crystallized-out phases 103 contain Si and Mg as additives, which are harder than the minute spacial areas within the dendrite cells 102.

In mirror finishing the surface of the blank 101 by the buffing, the buff coated with a compound buffs the surface in a state that the buff is in contact with both the minute spacial areas within the dendrite cells 102 and the crystallized-out phases 103.

As stated above, the crystallized-out phases 103 is harder than the minute spacial areas within the dendrite cells 102. Conversely, the minute spacial areas within the dendrite cells 102 is more soft than the crystallized-out phases 103. Accordingly, the efficiency of buffing the minute spacial areas within the dendrite cells 102 is different from that of buffing the crystallized-out phases 103. The finished surface of the blank 101 is shaped in a cross section such that the portions of the crystallized-out phases 103 are protruded, while the minute spacial areas within the dendrite cells 102 are curved downward to form depressions 104 (see FIG. 17). In the figure, S denotes a phantom straight plane S of the surface of the blank 101.

Therefore, it is seen that to reliably form a mirror surface by a mirror finishing, the distance or depth L ranging from the top of the crystallized-out phase 103 to the bottom of the depression 104 must be reduced.

In connection with this, reduction of the cell size D of the dendrite cells 102 leads to the reduction of the depth L. The cell size reduction is very effective in the case of forming the mirror surfaces of 1 μm or less in the surface roughness, particularly 0.4 μm or less. In this specification, the surface roughness means the center line average roughness Ra.

In FIG. 18, after a blank in which the cell size of the dendrite cells 102 is D1 is subjected to a predetermined mirror finishing, the crystallized-out phases 103 are left as stated above and large depressions 104a of depth L are formed at the dendrite cells 102.

When the dendrite cells 102 are reduced to have the cell size D2, which is smaller than the cell size D1, small depressions 104b of the same curvature as that of the large depressions 104a are formed in the spaces surrounded by the crystallized-out phases 103 since the mirror finishing tool, for example, a buff, is soft. In the case, the depth L2 of a the small depressions 104b is smaller than that L1 of the large depressions 104a.

Accordingly, the surface roughness of the mirror-finished surface must be improved by the extent of L1-L2 by reducing the size of the dendrite cells 102 of the blank 101, viz., reducing the DAS2.

On this presumption, the following confirmation test was conducted for evaluating the finishing quality of the surface finished by a mirror finishing work.

In the confirmation test, an aluminum wheel blank 11 (referred to as an aluminum wheel) as an aluminum alloy cast product was used as a test piece. The design surface 11a of the aluminum wheel 11 was mirror finished.

In this specification, the design surface 11a means a surface which faces outward from a vehicle body and forms a part of appearance of a vehicle when the wheel is mounted on the vehicle. An aluminum wheel 11 as a test piece was a cast product obtained by pouring a molten aluminum (for example, AC4CH) alloy into a mold having a predetermined shape. The cast product is subjected to a heat treatment (T6). The cast product was machined into a predetermined shape after removal of the mill scale.

Thereafter, the surface of the aluminum wheel 11, which includes the design surface 11a, was barrel worked in such a way as shown in FIG. 19.

In FIG. 19, reference numeral 21 designates a barrel polishing apparatus, and numeral 22 represents a work table 22. A push plate 24 is disposed facing an opening located at the central part of the work table 22.

The push plate 24 is driven to move vertically and angularly (as indicated by arrows marked in the central part in FIG. 19) in a reciprocative manner by a drive unit 23, thereby causing a barrel polished mixture 13 (to be described later) to be in motion.

The aluminum wheel 11 is fastened to the circumferential edge of the upper side of the hole of the work table 22. The aluminum wheel 11 is filled with the barrel polished mixture 13 formed by mixing work liquid, media, and compound at a proper ratio. A polishing chamber 12 is formed between the push plate 24 and the inner surface of the aluminum wheel 11. The surface of the aluminum wheel 11, which includes the design surface 11a, is barrel worked by driving the push plate 24.

In the barrel working of the aluminum wheel 11 as the test piece by the barrel polishing apparatus 21, the work liquid was water, the media was steel balls of the diameter approximately 0.8 to 1.2 mm, and the compound contained lubricating oil and soap. The amplitude and the frequency of the vertical and angular motions were 4.5 mm or less and 1000 to 2000 Hz.

The aluminum wheel 11 as the test piece was formed by barrel working the surface of the blank in this way.

Three aluminum wheels 11, which contain the dendrite cells of different cell sizes in the surface regions thereof, were formed as test pieces under different conditions of heat treatment.

To distinguish the different aluminum wheels 11 from one another, those were named as a test piece A, a test piece B and a test piece C.

The test piece A is an aluminum wheel in which the DAS2 in the surface region of the design surface 11a to be mirror finished was adjusted to a value of 30 μm in average. In the test piece B, the DAS2 was adjusted to 40 μm, and in the test piece C it was adjusted to 50 μm.

To check as to whether or not the DAS2 of the test pieces A to C were adjusted to the above figures, the surfaces of the test pieces A to C to be mirror finished were picked up at a plural number of locations thereon, to obtain samples. The metal structure of the surfaces of the samples were photographed by using an optical microscope. Using the photographs, DAS2 of the samples were measured according to the secondary technique of the dendrite arm spacing measuring procedure, which is employed in the Light Metal Academy.

The DAS2 measuring procedure used will be described with reference to FIGS. 16 and 20.

1) Portions to be measured are selected from the metal structure photographs.

In selecting the measured portions, at least three number of aligned dendrite secondary arms must be contained in each measured portion selected.

Accordingly, in the case of the metal structure shown in FIG. 16, a portion denoted as P is selected as the measured portion. A portion denoted as Q is unsuitable for the measured portion.

On the photograph, the length li between the boundary of the aligned dendrite secondary arm group and another boundary thereof in the measured portion that is selected on the basis of the recognition that the dendrite secondary arms or the arms recognized as the secondary arms are aligned.

Further, the intersection number ni when a line is drawn from the boundary of the aligned dendrite secondary arm group to another boundary thereof is counted (see FIG. 20).

In this way, the length li and the intersection number ni are measured, and the total number of dendrite secondary arms to be measured must be 30 or more. The measurement as mentioned above is carried out on the m number of the dendrite secondary arm groups satisfying the above conditions.

When the following equation is calculated using the thus measured values, we can obtain the secondary dendrite arm spacing d by the secondary technique. ##EQU1##

The thus calculated mean values of the DAS2 of the test pieces A to C were: 30.0 μm for the test piece A, 38.3 μm for the test piece B, and 49.0 μm for the test piece C.

As seen, the DAS2 values of those test pieces A to C are approximately equal to the aimed values.

Those three test pieces A to C were subjected to a predetermined mirror finishing by using the same buff and the same compound and under the same polishing load.

When the time for the mirror finishing is fixed, there is a fear that the finishing is influenced by the result of the barrel working as the preliminary step for the test pieces A to C. Accordingly, the time for the mirror finishing was not limited. That is, the mirror finishing work was terminated when the best finished surfaces of the test pieces A to C were obtained.

The surface roughness of the test pieces A to C thus mirror finished was 1 μm or less. Accordingly, the evaluation of the finished surfaces of the test pieces A to C was conducted by comparing them with the mirror surface samples, which were carefully formed using AC4C materials. The evaluation by the comparison was visually conducted by ten persons.

The results of the evaluation are tabulated in Table 2.

                  TABLE 2                                                          ______________________________________                                         observer    a     b     c   d   e   f   g   h   i   j                          ______________________________________                                         testpiece A 5     5     5   5   5   5   5   5   5   5                          testpiece B 4     5     5   5   5   4   4   5   5   5                          testpiece C 3     4     4   4   5   4   3   4   4   5                          ______________________________________                                    

The evaluation was based on a maximum of 5 points. Accordingly, in Table 1 , "5" indicates that the quality of the finished surface is comparable with that of the sample. "4" indicates that the former is slightly inferior to the latter. "3" indicates that the former is considerably inferior to the finished surface quality indicated by the point "4".

As seen from Table 1, all of the observers confirmed that the quality of the finished surface of the test piece A having the DAS2 mean value of 30 μm is comparable with that of the sample.

70% of the observer evaluated that the finished surface quantity of the test piece B having the DAS2 mean value of 40 μm is comparable with that of the sample, but the remaining 30% of the observers evaluated that the former is somewhat inferior to the latter.

The evaluation is based on the visual comparison by the observers. Accordingly, the evaluations by the observers are not invariable. When this fact is taken into consideration, it can be considered that the quality of the finished surface of the test piece B is also comparable with that of the sample.

20% of the observer evaluated that the finished surface quantity of the test piece C having the DAS2 mean value of 50 μm is comparable with that of the sample, but 60% of the observers evaluated that the former is somewhat inferior to the latter, and 20% of the observers evaluated that the former is considerably inferior to that of the sample. Accordingly, it can be considered that the finished surface quality of the test piece C is somewhat inferior to that of the sample.

As seen from the results of the evaluations of the test pieces A to C, the quality of the finished surface of the aluminum alloy blank is better as the mean value of the DAS2 is smaller.

From the table, it is further seen that if the cell size of the dendrite cell in the surface region of the aluminum alloy blank is adjusted to 40 μm or less in the mean value, the resultant mirror surface is extremely excellent, having the surface roughness 1 μm or less, particularly 0.4 μm or less.

Thus, when the aluminum alloy blank of the invention in which the mean value of the DAS2 in the surface region of the aluminum alloy is adjusted to 40 μm or less, is used, the work of finishing the design surface of the blank into an excellent mirror surface is more reliable than when the conventional blank is used. Accordingly, such an unwanted situation that the mirror finishing work of high working cost is brought to nothing, little occurs.

As seen from the foregoing description, the mean value of the DAS2 in the surface region of an aluminum alloy blank to be mirror finished is set to a value of not larger than 40 μm. Because of this, the spaces among the crystallized-out phases of larger hardness are reduced. In the surface region of the blank of which the mean value of the DAS2 is so selected, an excessive working of the dendrite portions is controlled to a minimum. Accordingly, the surface of the dendrite portions is a little curved downward to form shallow depressions, so that a reliable mirror finishing is ensured.

Accordingly, such an unwanted situation that the mirror finishing work of high working cost is brought to nothing, little occurs.

A vehicle wheel according to a further embodiment of the present invention will be described. The vehicle wheel is made of aluminum alloy, particularly aluminum alloy containing Si. The aluminum alloy vehicle wheel is a vehicle appearance component, which contributes to presenting a good appearance of a vehicle when it is mounted on the vehicle. The surface of the wheel, which faces outward from a vehicle body, is one of the key factors in vehicle design.

Various types of well designed uneven portions, such as spoke portions, are formed in a disk portion forming this design surface.

The vehicle wheel may be classified into a vehicle wheel of the type in which a wheel body and a rim portion are separately formed and then those are coupled into a one piece construction by means of welding, rivets, or the like, and a vehicle wheel of the one piece type in which the disk portion is integral with the rim portion. The one-piece type vehicle wheel is generally made of aluminum alloy containing Si and formed through casting.

The design surface of the one-piece type vehicle wheel has the uneven portions. Further, Si particles are crystallized out in the material. Accordingly, the design surface has a large surface roughness and poor gloss.

For the above reasons, the beneficial design features that the design surface of the aluminum alloy vehicle wheel potentially possesses have not been fully utilized.

When the vehicle wheel is mounted on the vehicle, the design surface faces outward from the vehicle body. Accordingly, it can receive light received from one side and reflect it toward the side. Accordingly, the design surface can improve the visual recognition when seen from the side of a vehicle. This feature of the design surface may be utilized for improving the safety of the vehicle. However, the level of a gloss of the design surface of the wheel is unsatisfactory for effecting such potential capabilities.

The present embodiment of the invention is capable of making full use of the useful design features of the design surface of the aluminum alloy vehicle wheel, and intensifying the reflecting light from the design surface, thereby to improve the visual recognition when seen from the side of the vehicle, and hence to improve the safety.

To this end, this embodiment is constructed on the basis of a technical idea that in an aluminum alloy vehicle wheel of the type in which the rim portion and the disk portion are coupled into a one-piece construction by casting, the surface roughness of the design surface, which is formed on the outer side of the disk portion, is set to 3 μm or less in the center-line average roughness.

Since the surface roughness of the design surface is set to 3 μpm or less in the center-line average roughness, the resultant design surface is more glossy than that of the conventional vehicle wheel. Accordingly, the potential useful design feature can be brought into full play, remarkably improving the appearance of the vehicle.

Further, the visual recognition when seen from the side of the vehicle is improved, thereby providing an increased safety of the vehicle.

This embodiment will be described in detail with reference to the accompanying drawings.

In FIG. 23, reference numeral 1' designates a vehicle 1'; numeral 2', a vehicle body 2'; 3', a front wheel 3'; and 4', a rear wheel 4'.

The front and the rear wheels are formed by fastening the vehicle wheels 5' (referred to as a wheel) of this embodiment on the axles (not shown). A tire 6' is mounted on the circumferential outer surface of the rim portions, which is also mounted on the circumferential outer surface of the wheel 5' of each of the front and rear wheels. The design surface in the disk portion of the wheel 5' is directed toward the outside of the vehicle body 2'.

The wheel 5' thus mounted on the vehicle 1' will be described with reference to FIGS. 21 and 22.

The wheel 5' is a called one-piece wheel formed by casting.

The wheel 5' is a cast product obtained by pouring a molten aluminum (for example, AC4CH) alloy into a mold having a predetermined shape. The cast product is taken out of the mold and subjected to a machining process after removal of gate portions. Through the machining process, a dimension accuracy required for the vehicle wheel is secured. Thereafter, the cast product is subjected to a process of barrel working, and the design surface 12' is finished by the buffing as will be described later. The thus finished surface of the design surface 12' is smooth and glossy.

The wheel 5', as the one-piece wheel formed by the casting, is formed of a ring-like rim portion 7' formed on the circumferential outer surface of the wheel 5', a hub portions 8', located at the central portion of the wheel 5', for fastening the wheel to the axle, and a disk portion 11' coupling the hub portion 8' with the rim portion 7'.

The outside surface of the vehicle disk 11' of the wheel 5' is formed to form the design surface 12' from a design point of view. The design surface 12' of the wheel 5' includes through holes 13' and spoke portions 14' which are shaped in conformity with a corresponding shape formed on the molding surface of the mold cavity when the wheel 5' is molded.

The surface of the design surface 12' of the wheel 5' has a complicated shape including variously curved portions and planar portions.

The design surface 12', well finished by the buffing, is an excellent finished surface having the surface roughness of 3 μm or less in the center-line average roughness Ra, and is more glossy than the conventional one.

The surface roughness of the design surface 12' is measured on a plural number of portions of the surface, which are considered as the average finished surface, and the arithmetic mean is used.

In order to reliably obtain a good finished surface of the center-line average roughness Ra 3 μm or less by a process of buffing on a level of the previous buffing, the secondary dendrite arm spacing (referred to as DAS2) in the surface region of the design surface 12' were measured. The mean value of the DAS2 was 40 μm or less.

The DAS2 measurement was conducted in the following way. The surface of the design surface 12' were picked up at a plural number of locations thereon, to obtain samples. The metal structure (FIG. 16) of the surfaces of the samples were photographed by using an optical microscope. Using the photographs, DAS2 of the samples were measured according to the secondary technique of the dendrite arm spacing measuring procedure, which is employed in the Light Metal Academy.

The DAS2 measuring procedure used will be described with reference to FIGS. 16 and 20.

In FIG. 16, reference numeral 101 indicates a photograph showing the metal structure on the design surface 12'; 102, a dendrite secondary arm; and 103, a crystallized-out phases corresponding to the Si particles.

When the mean value of the DAS2 in the surface region of the design surface 12' is 40 μm or less as stated above, a good finished surface having the surface roughness of 3 μm or less can be relatively easily formed by subjecting the design surface 12' to a predetermined process of buffing.

When the mean value of the DAS2 in the surface region of the design surface 12' is 50 μm or less, a finished surface having the surface roughness of 3 μm or less can be formed on a level of the reliability required in the present mass production technique. In this case, the reliability of forming such a finished surface is inferior to that of the 40 μm DAS2 case.

In the wheel 5' having the design surface 12' finished up to 3 μm or less in the surface roughness, the design surface 12' is more glossy than the conventional one.

Accordingly, the uneven design of the design surface 12' is emphasized by the gloss, so that the external design features of the design surface 12' are brought into full play. Accordingly, the external view of the vehicle 1' is made more excellent.

The reflecting light from the design surface 12' is intensified, improving the visual recognition when seen from the side of the vehicle. This feature implies that it can intensively reflect light from the head lamp of a vehicle 1' coming from the side at an intersection,particularly in the night. Therefore, it contributes to the safety of the vehicle.

As seen from the foregoing description, since the surface roughness of the design surface is set to 3 μm or less in the center-line average roughness, the resultant design surface is more glossy than that of the conventional vehicle wheel. Accordingly, the potential useful design feature can be brought into full play, remarkably improving the appearance of the vehicle. Further, the visual recognition when seen from the side of the vehicle is improved, thereby providing, an increased safety of the vehicle. 

What is claimed is:
 1. An electrolyte composite buff machining method comprising the steps of:providing a buff, having an electrode at one end of a rotary shaft, for buff machining a surface of a workpiece fixed to a machining table; supplying an electrolyte and an electrolyte current between the electrode and the workpiece for electrolyte machining, the electrolyte comprising a grinding powder; and at the same time, drivingly rotating the buff by the rotary shaft for buff machining, wherein said electrolyte composite buff machining method further comprises a step of, after said step of drivingly rotating the buff, forming a transparent coating film made mainly of inorganic material on the surface of the workpiece.
 2. An electrolyte composite buff machining method as recited in claim 1, wherein the buff is a soft buff, said method further comprising, at the same time as said step of drivingly rotating the buff, bringing a circumferential surface of the soft buff into contact with a surface of the metal molded article while supplying the molded article with a polishing liquid to which a polishing powder has been added.
 3. An electrolyte composite buff machining method comprising the steps of:providing a buff, having an electrode at one end of a rotary shaft, for buff machining a surface of a workpiece fixed to a machining table; supplying an electrolyte and an electrolyte current between the electrode and the workpiece for electrolyte machining, the electrolyte comprising a grinding powder; and at the same time, drivingly rotating the buff by the rotary shaft for buff machining, further comprising a step of:elastically retracting the electrode in an axial direction relative to the rotary shaft in response to an urging force on the buff.
 4. The method according to claim 3, wherein the electrode comprises a sheet material having an elasticity, and wherein the electrode extends within a plane substantially transverse to the axial direction of the rotary shaft.
 5. The method according to claim 4, wherein the sheet material comprises a mesh material.
 6. A method for finishing a surface of an article made of metal comprising the steps of:effecting a dry buffing to the surface of the article; after said step of effecting a dry buffing, effecting a wet buffing to the surface of the article; and effecting an electrolyte composite buff machining method, said electrolyte composite buff machining method comprising:providing a buff, having an electrode at one end of a rotary shaft, for buff machining the surface of the article; supplying an electrolyte and an electrolyte current between the electrode and the article for electrolyte machining, the electrolyte comprising a grinding powder; and at the same time, drivingly rotating the buff by the rotary shaft for buff machining, said method further comprising a step of:after said step of effecting a wet buffing, forming a transparent coating film made mainly of inorganic material on the surface of the article. 